1. Field of the Invention
The present invention relates to a gamma voltage generation circuit, and more particularly, to a gamma voltage generation circuit which is capable of synchronous level shifting of gamma voltages.
2. Description of Related Art
In the current information society, information dissemination media and various electronic display apparatus are being widely used in industrial use and home use devices, which makes the electronic display apparatus more and more important. The electronic display apparatuses have been continuously developed to meet various needs in the information society.
In general, the electronic display apparatuses display and deliver a variety of pieces of information to users using the information. That is, these electronic display apparatuses convert electronic information signals into optical information signals that are visually identifiable by the users.
In existing display apparatuses or systems, such as, a cathode-ray tube (CRT) display or a liquid crystal display (LCD), its input voltage and display output are in a non-linear relationship, and the relationship between the input voltage and the display output is described by a gamma curve. As far as the LCD is concerned, input voltages (i.e., gamma voltages) for corresponding grey levels can be found using the gamma curve. Using these gamma voltages to control the LCD panel to display correct grey levels, the LCD can correctly display images.
To improve display effects of the LCD, one pixel consists of two sub-pixels in some LCD panels. The level of the common voltage for the two sub-pixels may vary due to the pixel circuitry variation. In this event of different common voltage, when the same gamma voltage is used to control the LCD panel, the two sub-pixels may display differently thus affecting the displaying quality. Therefore, the level of the outputted gamma voltages may be different to allow the different sub-pixels to display the same effect. In other words, in order for some pixels to display the same effect, a level-shifted gamma voltage must be received.
FIG. 1 is a circuit diagram of a conventional gamma voltage generation circuit. Referring to FIG. 1, the voltage between a reference voltage AGMAH and a reference voltage AGMAL is divided by resistors AR0-AR63, and gamma reference voltages AV0-AV63 are therefore outputted. The voltage between a reference voltage BGMAH and a reference voltage BGMAL is divided by resistors BR0-BR63, and gamma reference voltages BV0-BV63 are therefore outputted. Switches 110_1-110_64 select and output the gamma reference voltages AV0-AV63 as gamma voltages V0-V63 or output the gamma reference voltages BV0-BV63 as the gamma voltages V0-V63 according to a control signal S1. A digital-to-analog converter 130 selects and outputs one of the gamma voltages V63-V0 as a driving voltage.
FIG. 2 is a circuit diagram of another conventional gamma voltage generation circuit. Referring to FIG. 2, the voltage between a reference voltage GMAH and a reference voltage BGMAL is divided by resistors CR0-CR64, and gamma reference voltages CV0-CV64 are therefore outputted. The gamma reference voltage CV1 is equal to a reference voltage AGMAL and the reference voltage AGMAL is greater than the reference voltage BGMAL. Switches 210_1-210_63 selects and outputs the gamma reference voltages CV0-CV63 or CV1-CV64 as the gamma voltages V0-V63 according to a control signal S1. A digital-to-analog converter 230 selects and outputs one of the gamma voltages V63-V0 as a driving voltage.
Although capable of tuning the level of the gamma voltages V0-V63, the above-described circuit utilizes multiple switches to select the gamma reference voltages. With the increase of the number of the switches, the circuit design becomes increasingly complex and the hardware cost is increased as well.